Photographs of performing artists can be found in a number of places...
Websites will often have photos of individual artists or groups, especially on the "official" website of the artist.
Books, both print and ebooks, sometimes include photos of artists or groups. You can search our catalog for autobiographies or documentaries about an artist or group to see if there is anything you might be able to use.
Sometimes you may find the best photos onine in a library collection or an archive! For example, The Grateful Dead Archive is located in the University of California, Santa Cruz Libraries, and there are some photos available online. The Rock and Roll Hall of Fame also has a library and archives with some materials available online.